1. Field of the Invention
This invention relates to an improved electrical double layer capacitor. More particularly, this invention relates to a novel process for balancing the leakage current of individual cells when stacked in series in an electrical double layer capacitor.
2. Description of the Art
This invention has two advantages over the prior art. First, this invention provides a means for controlling the gas evolution when charging double layer capacitors. Second, it provides a means for balancing the voltage and leakage current of individual capacitor cells when charged in series.
The evolution of gas when charging a capacitor is a serious problem. In sealed cells, the gas evolution can cause rupture of the invididual cell cases while in vented cells, where the cells are enclosed within a closed system, gas evolution can cause rupture of the container or even produce explosive atmospheres of various gas mixtures within the container.
Variances in the leakage current of capacitor cells is a significant problem when stacking individual cells in series. It is necessary to stack individual cells in series to increase the total voltage output of a capacitor since every such capacitor cell has a fixed voltage rating. Moreover, since the same amount of current will flow through each cell when stacked in series, the overall voltage output of a capacitor composed of a plurality of individual cells should theoretically equal the sum of the separate voltage capabilities of the individual cells. Thus, in order to maximize the efficiency, capacity and voltage output of a capacitor, the voltage characteristics of each cell should match since a defective cell can limit the voltage output of the entire capacitor.
However, individual cells tend to become unbalanced when charged in series. Some cells have low leakage currents while other cells have high leakage currents. If current is continued until all the cells are charged, those with low leakage currents will overcharge experiencing gas evolving reactions while other cells are still below the decomposition voltage of the electrolyte. As an example of gas evolution in a cell, during the overcharge of an aqueous electrolytic cell, oxygen will be formed at the positive electrode while hydrogen is formed at the negative electrode.
Leakage current as used herein, is the continuous current that passes through a fully charged capacitor cell to maintain steady state voltage conditions. In other words, leakage current can be measured by the amount of current required to maintain a constant voltage in the cell after full charge is obtained.
Previously used techniques for controlling these problems have not proven entirely satisfactory. Typically, capacitors are protected from overcharge by limiting the extent of charge. For example, conventional capacitors are charged to a voltage significantly below their total capacity. Thus, even when the voltage across any single cell is significantly increased to compensate for a less efficient cell, the possibility of overcharge is greatly diminished. However, in order to accomplish this, the capacitor is conventionally charged to approximately only 50-60% of the amount that is theoretically possible calculated from the voltage ratings of the individual cells. For example, a capacitor having three unit cells theoretically capable of being charged to three volts (one volt per cell) is charged only to between 1.5 and 2 volts. It is clear that this technique is both economically and electrically inefficient since additional unit cells must be used to obtain a specific total voltage output.